This invention relates generally to the disinfection of water by chlorine dioxide, and more particularly to a system adapted to continuously monitor or analyze the concentration of chlorine dioxide dissolved in water used in industrial process solutions, municipal water supplies and in other applications requiring disinfected water, the system including a sensor immersed in the liquid to afford an in situ analysis thereof.
The virtues of chlorination techniques for disinfecting water supplies has long been recognized, but only in recent years hve the hazards involved come to public attention. Thus in studies conducted on the chlorinated water supplies of the city of New Orleans, it was found that the levels of chlorination were such as to release carcinogenic agents dangerous to the community.
The results of this study appear in an article by R. A. Harris, "The Implication of Cancer Causing Substances in Mississippi River Waters" published Nov. 6, 1974 by the Environmental Defense Fund, Washington, D.C. Also relevant in this regard is the article of A. A. Stevens et al., "Chlorination of Organics in Drinking Water," Proceedings of the Conference on the Environmental Impact of Water Chlorination--Oak Ridge National Laboratory, Oak Ridge, Tennessee, Oct. 22-24, 1975 (Journal AWWA 68:615-1976).
Chlorination has been found to result in the formation of chlorinated organic compounds--trihalomethanes. Analytical studies of source and finished waters have demonstrated that trihalomethanes are widespread and arise as a direct result of the chlorination practice. It has also been shown that the organic precursors to trihalomethane formation are the natural humic materials present in almost all source waters.
The disinfection of water with chlorine dioxide rather than chlorine in order to circumvent the formation of trihalomethane carcinogens is now well known. Chlorine dioxide does not produce trihalomethanes in water treated thereby. Moreover, it reacts to reduce the concentration of the precursor such that if chlorine is also used as a disinfectant, the resulting trihalomethane produced is diminished.
Chlorine dioxide gas has a greenish yellow color similar to chlorine gas, but is more toxic. Because of its explosiveness, chlorine dioxide is generally used as a disinfectant in a non-hazardous aqueous solution, for it is extremely soluble in water, about five times more so than chlorine.
The main concern of the present invention is with the continuous monitoring or analysis of chlorine dioxide dissolved in water. In any municipal, industrial, or other process which makes use of this disinfectant, it is vital that the concentration of the solution be accurately determined in order to effect proper regulation of the amount of chlorine dioxide introduced into the process. Thus in an automatic control system for governing a process making use of chlorine dioxide dissolved in a process liquid, one must be able to continuously sense the existing concentration of the dissolved chlorine dioxide (DCD) in order to compare this process variable with a set point to generate a control signal for regulating the process.
Presently known procedures for the continuous analysis of chlorine dioxide concentrations leave much to be desired. Existing analytical techniques described in the chemical literature are either batch procedures which are unsuitable for automatic process control or involve flow-through optical procedures employing spectrophotometry.
The problem of chlorine dioxide analysis is complicated by the presence in most waters of residual chlorine (free and combined) and chlorite as well as chlorine dioxide. Since chlorine dioxide reacts in a manner similar to chlorine with reagent chemicals, with conventional analytic techniques, one cannot differentiate between these distinctly different disinfectant agents.
A widely used technique to overcome this problem is the method described by Haller et al. in Anal. Chem. 31, 872 (1948) in which penylarsene oxide is used as a titrant for the determination of free chlorine, combined chlorine, chlorite and chlorine dioxide in any combination. But this technique is time-consuming and difficult to carry out.
The spectrophotometric approach lends itself to continuous analysis but is subject to serious error caused by contamination of the optical windows. And because at low levels of DCD concentrations, color-developing reagents must be added to enhance the sensitivity of the optical analyzer, this dictates the inclusion of microflow pumps to deliver the reagent. The need to maintain these pumps as well as to replenish the reagents which are consumed in the course of testing represent practical drawbacks.
Inasmuch as the present invention makes use of a DCD sensor which is structurally similar to the dissolved oxygen probe disclosed in the Poole U.S. Pat. No. 3,948,746, this patent is made of record herein even though the Poole probe is incapable of analyzing dissolved chlorine dioxide, for the diffusion membrane incorporated in this probe is not permeable to chlorine dioxide.